This invention relates to a separation type optical pick-up device adapted to correct a discrepancy between an optical axis of a light beam emitted from an emitting portion of a stationary optical unit and an optical axis of a light beam returned from a movable optical unit.
An optical pickup device is known as a device for recording or reproducing information into or from an optical information recording medium such as an optical disk and various types of the device have been put to practical use.
In a conventional optical pickup device, an access operation and a tracking operation are carried out by providing the whole optical pickup device adjacent an optical information recording medium and positioning at the track on the medium so that the optical pickup device in weight is an obstacle for the increase in access speed.
In consideration of this, recently, a separation type optical pickup device, which is an optical pickup device divide into two portions, one for a stationary optical unit and the other one for a movable optical unit, operable to access by fixing the stationary optical unit and moving the movable optical unit has been proposed.
The stationary optical unit for the separation type optical pickup device includes means for emitting light including a semiconductor laser, and signal detecting means for detecting track error signal, focus error signal and RF signals. The movable optical unit also includes an object lens, a focus actuator, a tracking actuator, and a deflecting member. The movable optical unit may include tracking mirror means instead of the tracking actuator and the deflecting member.
The focus actuator is provided to move the object lens in the axial direction of the laser beam, whereby the laser beam from the stationary optical unit is focused properly onto the recording surface of the optical information recording medium.
The tracking actuator is also provided to move the object lens in the direction of crossing at right angles with the optical axis of the laser beam, whereby the focused position of said laser beam is not turned from the track.
The tracking mirror means is provided for the tracking operation by which the direction of the laser beam incident on the object lens is changed by changing the inclination of the tracking mirror.
The optical axis of the object lens shifts in parallel with the optical axis of the incident light beam from the stationary optical unit, when the position of the object lens is moved in the direction of crossing at right angles with the optical axis of the laser beam with the tracking actuator. On tracking operation by the tracking mirror system, the optical axis of the incident light beam shifts at an angle with the optical axis of the object lens.
Such a shift between the optical axes causes a discrepancy between the optical axis of laser beam emitted from the stationary optical unit and the optical axis of light beam which is reflected by the optical information recording medium and coming into the stationary optical unit again through the movable optical system. The discrepancy causes an error in focus error signals, a change in the diameter of the light spot on the optical information recording medium, and inconvenient states in that a direction for the movement of the object lens in the tracking direction goes over the movable range of the tracking, and in that a direction for the rotation of the tracking mirror in the tracking direction goes over the movable range of the tracking mirror rotation.
As a method of solving such problems, it has been intended that the discrepancy of the optical axes is minimized by loading light detecting means in combination with LED (light emitting diode) and light receiving element on the movable optical unit, detecting the position of the tracking direction for the object lens, detecting the inclination of the tracking mirror and moving the movable optical unit by drive means based on the result detected to keep the normal position between the optical recording medium and the movable optical unit.
However, this method includes several problems in that there is a increase in weight due to the optical detecting means loaded on the movable optical unit and it reduces one of the advantages of the separation type optical pickup device, and also the optical detecting means is easily affected by noises accompanied with the movement of the movable optical unit, therefore, there is also another problem in reliability.
Furthermore, recently, provision of the tracking mirror on the stationary optical unit has been intended to make the best use of advantages of the separation optical pickup device. However, such provision does not solve the problems in the discrepancy of the optical axes and also optical detecting means for detecting the inclination of the tracking mirror needs to be provided on the stationary optical unit. On this type of optical pickup, loading said optical detecting means on the stationary optical unit has been a chief problem in designing, and further more problem is that the optical axis is not properly corrected only by detecting the inclination of the tracking mirror because the precision in detecting the inclination of the tracking mirror changes in response to the seek position of the movable optical unit.